Release Candidate builds

In preparation for the upcoming release of 3.12, release candidate
builds are available.

Unfortunately the current release of Rockbox Utility doesn't know what
to do with them. A new release will be made soon, but for now
development builds of Rockbox Utility are available for Windows and OSX
here: http://www.alice-dsl.net/dominik.riebeling/rockbox/rbutil/ Linux
users will need to build it themselves.